Abstract: Polyethylene glycol (PEG), due to its ability to resist protein adsorption and reduce RES clearance, has been widely used to extend the circulation times of protein and nanoparticle therapeutics. Unfortunately, recent animal and human studies have shown that anti-PEG antibodies (APA) can be either induced by select PEGylated therapeutics, or are pre-existing, presumably due to constant exposure to PEG in everyday hygiene, skincare and food products. In turn, APA can substantially limit the circulation kinetics of PEGylated therapeutics and render them non-efficacious, and potentially even unsafe. In light of the increasing number of PEGylated therapeutics that are either FDA approved or in clinical development, there is an urgent need for interventions that can enable the use of PEGylated drugs in patients with high APA titers. Here, by combining physiology-based pharmacokinetic (PBPK) modeling with a series of in vitro and in vivo studies, we have identified administration of high MW free-PEG to be a simple, effective and safe strategy in blocking APA binding to PEGylated therapeutics. Indeed, injection of free-PEG increased the fraction of circulating PEGylated liposomes in mice with substantial levels of APA by >100-fold compared to PBS control, reaching levels identical to mice without APA. More importantly, all toxicity studies of this intervention, including complete blood counts, liver and renal functions as well as careful tissue histology, were indistinguishable from PBS control and showed no evidence of glomeruli inflammation. The surprising, complete lack of toxicity appears related to the highly flexible nature of PEG coupled to the absence of PEG anchoring to substrates with fixed conformations, which limits the formation of immune complexes and triggering conventional effector functions. To realize the full potential of this strategy, we propose a rigorous research program that combines in silico, in vitro and in vivo approaches to evaluate the effectiveness and safety of the use of free-PEG in mice with high titers of APA for two PEGylated therapeutics, Krystexxa (PEG-uricase for treatment of gout) and Onivyde (PEGylated liposomal irinotecan for treatment of metastatic adenocarcinoma of the pancreas). In Aim 1, we will further develop our PBPK model to guide the optimization of use of free PEG (PEG MW, dose, dosing regimen, etc) to suppress APA, and to predict the effectiveness of this intervention in both mice and humans. In Aim 2, we will verify the predictions from Aim 1 for restoring prolonged circulation of Krystexxa in mouse with APA titers matching those found in humans through a series of carefully designed pharmacokinetic, biodistribution and toxicity studies. Finally, in Aim 3, we will verify the predictions from Aim 1 for restoring the effectiveness of Onivyde in a genetically engineered mouse model of pancreatic cancer with APA. If successful, our work will identify a readily translatable pathway for restoring the use of a variety of PEGylated agents in patients with high titers of APA.